13.2.11.Table snapshot logs

In order to be able to create incremental snapshots on a table there should be a snapshot log table defined in the following fashion:

create table RLOG_sourcetable (

The name of the snapshot log table is constructed from the name of the source table by prefixing it with RLOG_ .

Snapshot log table contains all the primary key parts of the source table prefixed with RLOG_ .

Snapshot log table contains at most one row per updated source table row with the update time and update type in DMLTYPE (I - for insert, U - for update, D - for delete).

Snapshot log table contains RLOG_ROWGUID column which uniquely identifies this snapshow log row.

An updating procedure for incremental snapshot or bi-directional snapshot (table case) reads records from snapshot log table, ordered by SNAPTIME. Commits are performed each time an updating procedure notices that SNAPTIME has changed. In Virtuoso case (when native table is snapshot-replicated) all snapshot log records with equal SNAPTIME belong to the same transaction and all such records constitute almost a transaction (it is not exactly a transaction because only the last DML operation for each primary key in the source table is recorded in snapshot log table). So in Virtuoso case an updating procedure commits almost per transaction on source table. Snapshot log records are not transaction-bound in heterogeneous case however. Nevertheless, the technique used to determine when to commit described above prevents running out of transaction log or deadlocks even in heterogeneous case.

Replay log is created on the server which performs sync (server on which snapshot log was create in ordinary case, publisher in bi-directional snapshot case) to handle correct snapshot log replaying. Replay log is purged periodically using REPL_PURGE_RPLOGS() function in ordinary case and REPL_PURGE_URPLOGS() in bi-directional case. The calls to this functions are automatically scheduled when snapshot log or bi-directional snapshot publication is created and there is no need to call them manually.

There should be triggers on the source table to update the log like that:

create trigger xxx_I_log after insert on xxx
  insert replacing RLOG_xxx values (xxx_pk, now(), 'I', uuid());

create trigger xxx_D_log after delete on xxx
  insert replacing RLOG_xxx values (xxx_pk, now(), 'D', uuid());

create trigger xxx_U_log after update on xxx
  if (OLD.xxx_pk differs from NEW.xxx_pk)
      insert replacing RLOG_xxx values (OLD.xxx_pk, now(), 'D', uuid());
      insert replacing RLOG_xxx values (NEW.xxx_pk, now(), 'I', uuid());
    insert replacing RLOG_xxx values (xxx_pk, now(), 'U', uuid());

where xxx is the source table name, RLOG_xxx is the snapshot log table name, xxx_pk is a comma separated list of the source table's primary key parts. The names of the triggers are constructed from the fully qualified name (DB.DBA.Orders makes DB_DBA_Orders_I_log).

Table snapshot logs and triggers are created automatically for Virtuoso and databases listed in Heterogeneous Snapshot Replication section.